There are two possible issues, and each issue will be different based on your OS.

Droid: Google likes to "randomly review" your Google Play account. It seems they have a background process that will eventually nab everyone. In short, they block repeated purchases on your Google Play account (regardless of source - Google Wallet, or CC.) If you attempt to make 2 purchases back-to-back, they will flag you. You can submit a ticket, but it's faster to call them. There are two different divisions - Google Wallet and Google. Google Wallet is limited in what they can do and what they will explain to you. They'll tell you to update your address and fully complete your Google Wallet profile. They'll also tell you to wait 5-10 minutes between in-app purchases. None of that prevents the error. There is a listing on your Google Wallet page for transactions. If you have a transaction listed as PENDING, it will probably block any additional purchases until that pending transaction is listed as completed. This is not sure-fire and foolproof, though, and doesn't necessarily mean that you won't get the error. 90% of this is Google's doing. The other 10% is from Mobage and they aren't saying anything other than pointing at Google if it's their fault or promising to follow-up if it's on Mobage. The most you'll get out of Mobage is "we're looking into it," or "your issue has been resolved." Google takes about 8-10 hours to get back to you unless you call them directly. Mobage takes 12-24 hours to respond to you. I am not aware of a support phone line for Mobage.

iOS - Apple Store has a very similar series of safety checks in place and will also place your account in review without warning, explanation, or details. You can call the iTunes support line or you can email. The Apple issue was resolved without any follow-up email from them (or response at all) after 48 hours.

In short, if you attempt to make a last minute purchase in-app t try and get that last little edge at the tail end of an event, you're at risk of account review due to the repeated purchases. There is no way around it and I pressed Google for answers and explanations about why this happens. Even the supervisor I spoke with was vague and limited by what they would disclose. It's clearly an anti-fraud policy, but they advised that the fastest way for Google to resolve such an issue in a time-sensitive manner is to call them directly (they're available 24-7.)

This also applies to purchases in which you placed your order but did not receive the content. Take screenshots of the process, and cross reference those with your Google Wallet transactions. Contact Google first, and then contact Mobage and supply the information. Mobage will be able to match up you transaction, time stamp, and game state to see if you were not given the items you purchased. I have not yet read of an issue like this where Mobage hasn't reconciled the transaction given proof.

Again, most responses to issues via email are likely to take as long as 48 hours, if not more.